Despite their importance in the pathogenesis of viral illness, specific receptors have been identified for only a few of the viruses that cause human disease. We have identified VLA-2, a member of the integrin family of adhesion receptors, as the receptor for echoviruses 1 and 8. More recently, we found that Decay Accelerating Factor (DAF, CD55), whose normal function is to protect cells from lysis by autologous complement, is a receptor for echoviruses 6 and 7, and for other serotypes that do not bind VLA-2. The overall goals of our work are: first, to define structural features of both receptor and virus responsible for their physical interactions; and second, to understand how receptors function in the process of infection. Although it has been suggested that a number of viruses bind integrin molecules, VLA-2 is the only integrin known to serve as a virus receptor. A detailed knowledge of VLA-2 interactions with echovirus 1 may serve as a model for how other viruses interact with this large and important class of cell surface molecules. CD55 is the first identified virus receptor anchored in the membrane by a GPI-linked fatty acid tail rather than a transmembrane peptide domain. Because GPI-ancored proteins are excluded from clathrin-coated pits, and may internalize ligands without detaching from the cell surface, the possibility exists that viruses that bind DAF may enter cells by a novel route. In the proposed experiments we will identify the structural features of both receptors responsible for virus binding. We recently showed that echovirus 1 binds to the I domain of the human alpha-2 subunit of VLA-2, and we will localize the E7 binding site on DAF using already available deletion mutants. We will trace the paths by which echoviruses 1 and 7 enter cells, to learn whether different types of receptors target viruses to different intracellular sites. We will determine whether the virus and receptor are internalized together, and whether virus binding triggers receptor internalization. In addition, we will analyze whether contact with the receptor initiates virus uncoating. To begin to understand the viral structures responsible for interaction with the receptor, we will sequence the capsid proteins of echovirus 1, and, in collaboration with Dr. James Hogle, determine the virus structure by X-ray diffraction. Knowledge of the virus structure, and the isolation of viral cDNA, will facilitate future genetic experiments to identify the viral sites for receptor binding.